CIRCULATION 



20i 



thick that the deeper lying cells would be starved by such a 

 method of food distribution. Moreover, the wall of the di^^'sti ve 

 tract is so thick that it would greatly impede the transfer of 

 absorbed food to the body fluid. There is, therefore, necessary 

 a system of channels by which the food may more readily be 

 transferred from the seat of digestion to the place of assimila- 



A 



ii«C7^ 



^C*X. 



D.V, 



Fig. 113. — The circulatory system of annelids. A, A longitudinal section of 

 a blood-vessel of a small fresh-water annelid (Chaetogaster) showing extremely 

 thin walls. B, Cross section diagram of nereis to show the arrangement of the 

 vessels; D.V., dorsal vessel; /«/, intestine; A^. nephridium; P.V., jiarapodial ves- 

 sels; V.I., intestinal vessels and capillaries; V.V., ventral vessel. All vessels 

 black. 



tion. These channels consist of a network of tubes of extremely 

 small calibre, which penetrate to every part of the intestinal 

 wall, immediately outside the intestinal epithelium. Larger 

 vessels lead from this network of capillaries to a much larger 

 vessel which runs longitudinally along the mid-dorsal line of 

 the body. In each segment branches of the dorsal vessel lead 

 out laterally to the muscles, epidermis and all other organs of 



